The field of the invention relates generally to spacecraft control, and more specifically, to a method and systems for automatically damping nutation and removing wobble from a spinning spacecraft.
The stability of a satellite rotating about a given axis is a concern in many known aerospace applications. For example, some known spacecraft, such as geosynchronous communication satellites, spin about a geometric axis during transfer orbit. More specifically, while in the transfer orbit the spin of a satellite must be stable so that procedures such as attitude determination, thermal control, propellant management, fuel-efficient velocity increment maneuvers, command and telemetry linkage and solar power collection can be accurately performed. A number of disturbance torques, which can be caused by incremental velocity or spin speed change maneuvers, may alter the attitude of the spacecraft. More specifically, these disturbances can induce nutation in the spacecraft. Furthermore, a mass redistribution during the transfer orbit spin can induce wobble in the spacecraft. For example, while in a transfer orbit, when the spin of a satellite is about an intermediate inertia axis, i.e., an axis having a moment of inertia thereabout less than the moment of inertia about a maximum principal axis, and greater than the moment of inertia about a minimum principal axis, the resulting spin is highly unstable. Specifically, a rapidly growing exponential divergence is produced in an uncontrolled intermediate axis spin, as opposed to the slowly-growing divergence which occurs in nutation.
Some known approaches to nutation damping include active nutation damping, wherein two momentum wheels and two gimballed momentum wheel platforms are used in a “vee wheel” configuration. The momentum wheels and platforms are employed to enhance the spin momentum and make the spin axis appear to have the maximum moment of inertia. A problem with these known nutation damping approaches is that they do not account for wobble control. Some other known systems employ an active spin-axis control system to stabilize the intermediate axis spin using thrusters in combination with gyro rate sensing. However, such systems consume irreplaceable propellant when using the thrusters, and further, the orbit and momentum of the satellite are disturbed by use of the thrusters.
In some known spacecraft control systems, a wobble control assembly has been used by open loop dynamic balancing which requires an estimate of the mass properties of the spacecraft, in-flight iteration, and adjustment after major maneuvers. Such spacecraft systems typically require a large number of spin maneuvers to reduce inertia ratios and the geometric wobble angle may substantially change during the transfer orbit. A problem with open loop dynamic balancing is the necessity of iterative ground procedures for controlling the wobble. The success of a mission requires a stabilized spin, effective nutation damping, and wobble control.